UPDATE 1-U.S. Vice President Biden visits Baghdad

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By Andrew Quinn

BAGHDAD, July 2 (Reuters) - U.S. Vice President Joe Biden, on a previously unannounced visit to Baghdad on Thursday to meet Iraqi leaders, U.S. military commanders and troops, said he was optimistic about Iraq's future but much work remained.

Biden's three-day visit comes after President Barack Obama appointed him to help coordinate Iraq policy as U.S. officials lay the groundwork for a full withdrawal of U.S. forces by 2012.

"This is a moment when we have to make sure that the Iraqis don't take their eye off the ultimate prize," Biden said.

White House officials said Biden would meet President Jalal Talabani and Prime Minister Nuri al-Maliki as well as visit U.S. commanders and troops, marking the U.S. July 4 Independence Day holiday.

Obama visited Iraq in April.

Officials said Biden might meet up with his son, Beau Biden, who is in Iraq with the Delaware National Guard in a unit that was activated for deployment late last year. (Editing by Frances Kerry)